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Appendix VI.                          1161

Electric ignition, pp. 700 and 963, is now almost universally
adopted, being more reliable with gas than with oil engines,
especially when scavenging is adopted.

Modern Engine Examples. The 500 H.P. Premier Engine,
Fig. 1025, may be taken as a good example of the larger kind.
There are two pistons A and B connected up in tandem to one
cross-head c, which forms also a piston for air compression; and
while A is coupled direct, B is driven by side rods, stretching from
c to j, outside the cylinder. Air is admitted by the gridiron valve
F, and compressed by the piston c into the passage H, from which
it is drawn, both for charge mixture and scavenging, through the
valve chambers D D. The exhaust valves E E, and the pistons A B,
are all cooled internally. The valve chambers are shewn in the
enlarged section, N being the main admission valve for the
mixture, spring closed and lever opened at P, and R a combined
air and gas valve, whose. opening is regulated by the governor.
When the latter valve is opened to gas, the air ports are partially
closed, for the full air opening is only required on the scavenging
stroke, and the mixture is so controlled as to fire easily at all
conditions of load. The explosions occur in each cylinder
alternately, or one per revolution of the crank.

The Westinghouse Engine, Figs. 1026 and 1027, is built in
units of two or three (up to 650 H.P. and over), working on the
same crank shaft, the latter form being illustrated. This method
of driving produces a more regular turning moment, and makes
the engine specially suited for lighting stations. The stress
variations are, however, large, and necessitate a much stronger
shaft than usual. Fig. 1026 is a section through one of the
cylinders, shewing a well-designed connecting rod and balanced
crank, and a cooled cylinder head. The half-speed shaft R is
provided with a cam to life the lever Q, which raises the rod H
against a spring N, and opens the exhaust valve L, allowing the
products to escape by the exhaust pipe K. Another shaft D, also
rotating at half-speed, opens the inlet valve E, by the cam lever
o, while the gas and air enter the mixing chamber M through the
pipes A and B respectively. The mixture is throttled by the
governor to suit varying loads, by means of the lever c; which
raises the throttle valve within M ; and the ignition is obtained